Many semiconductor imaging sensors today are front side illuminated. That is, they include imaging arrays that are fabricated on the front side of a semiconductor wafer, where light is received at the imaging array from the same front side. However, front side illuminated imaging sensors have many drawbacks, one of which is a limited fill factor.
Backside illuminated imaging sensors are an alternative to front side illuminated imaging sensors that address the fill factor problems associated with front side illumination. Backside illuminated imaging sensors include imaging arrays that are fabricated on the front surface of the semiconductor wafer, but receive light through a back surface of the wafer. However, to detect light from the backside, the wafer must be extremely thin. Color filters and micro-lenses may be included on the back surface of the wafer in order to improve the sensitivity of the backside illuminated sensor. The thickness of the wafer may be optimized in order to improve the sensitivity and reduce crosstalk. However, higher sensitivity typically results in higher crosstalk. That is, as the final semiconductor wafer thickness is increased, light can be more effectively collected by the wafer. At the same time, light intended for one pixel may have a higher possibility (or greater chance) of reaching other pixels that were not intended to receive the light. Thus, a need exists for a backside illuminated device with improved sensitivity that reduces crosstalk.